Solid ammonium nitrate explosives and propellants



Sept. 24, 1963 N. J.YBOWMAN 'ET'AL 4 sous AMMONIUM NITRATE EXPLOSIVES AND PROPELIIJANTS Filed June 20, 1952 S: & s

0 90 5 mm N mww m #9 A m A 8 m 00 NW Y B a (m w United I States Patent 3,104,994 SOLID AMMONIUM NITRATE EXPLOSIVES AND PRUPELLANTS Norman J. Bowman, Hammond, Ind, and Wayne A.

Proell, Chicago, Ill., assignors to Standard Oil Company, Chicago, Ill, a corporation of Indiana Filed June 20, 1952, Ser. No. 294,528 11 Claims. (Cl. 149-19) This invention relates to a composition for the generation of gas, which composition can be used as an explosive or propellant. More particularly, the invention relates to explosive compositions wherein ammonium nitrate is substantially the only explosive agent. Still more particularly, the invention relates to a shaped composition for the generation of .gas by the decomposition of a mixture comprising essentially ammonium nitrate, oxygenated oxidizaible material and I8, catalyst. Also, the invention relates to a novel composition usable as a binder for a shaped explosive. Further, the invention relates to methods for the preparation of said binder and said shaped explosive composition. Also, the invention relates to a method of propelling rockets or assisting in the propulsion of aircraft.

Ammonium nitrate is widely used as a component of high explosives, particularly the so-called safe explosives. Even though ammonium nitrate is classified as a high explosive, it is extremely insensitive to ordinary heating and to shock and cannot readily be detonated by the local application of heat or by a blasting cap. Further, when ignited, ammonium nitrate alone does not burn uniformly and has a tendency to go out. In order to improve the burning quality to increase the sensitivity and to utilize the excess free-oxygen available from the decomposition of the ammonium nitrate, oxidizable materials, such as,

carbon, cellulosic materials, hydrocarbons, etc. are ladmixed with the ammonium nitrate.

World War II utilized in tremendous quantity rockets for ground-to ground missiles, ship-to-shore missiles, airto-ground and air-to-air missiles. These rocket-s comprised essentially a thin-walled casing which contained a combustion chamber containing aquantity of solid propellant, a nozzle through which the decomposition gases passed and created the forward thrust, stabilizing fins and a war head which contained the explosive. The military rockets utilized the so-called double base powders such as Ballistite as'the solid propellant. Also, there was developed the use of rocket units to assist in the take-ofi of either heavily ladened airplanes or to overcome a short runway. These units are commonly known as JATO (jet assisted take-oil) or ATO (assisted takecfi) units.

The use of ammonium nitrate-base compositions as solid propellants for rockets and ATO units is quite attractive because of the cheapness and availability of ammonium nitrate; because of the relatively low flame temperature of the decomposition of ammonium nitrate, between about 3150 and 3900" F. (l730-2l50 C.); and because the excess free-oxygen available from the decomposition permits the use of oxidizable material to improve the energy obtainable from the decomposition. However, it was found that the physical characteristics of ammonium nitrate seriously interfered with the development of ammonium nitrate-base solid propellants.

Solid ammonium nitrate can exist in five difiierent EEOFIIIS. These forms are stable within certain temperature ranges, yet pass readily into the form stable at a different range when the temperature of the solid is brought to the transition temperature or a degree or two beyond the transition temperature. Each phase change is accompanied by a change in the volume occupied by a unit weight of the ammonium nitrate. There are given below data on the transition temperatures of the various known solid forms ddd ifi d Patented Sept. 24, 1963 of ammonium nitrate and also the approximate volume change in termsof percent.

Temperature 1 Volume Phase change 1 change,

percent 2 0. F.

1 R. G. Ear1y-'l. M. Lowry, J. Chem. Soc. 115, 1187 (1919). 2 International Critical Tables.

used to form a shaped solid propellant (grain). However,

no successful ammonium nitrate base grain has been developed by other workers in this art. Binders which produced a grain that resisted the formation of cracks (fissures) were sufficiently thermoplastic that the grain flowed at higher atmospheric temperatures and became deformed; such a thermoplastic grain cannot be used because its ballistic characteristics are not predictable. Other binders which gave grains that were dimensionally stable at these temperatures were unable to withstand the volume change occur-ring during the transformation of the ammonium nitrate from one form to another; these grains developed fissures both entirely internal .and/ or extending to the surface of the grain; such fissures act as burning surfaces and change the burning characteristics of the grain, thus making the ballistic performance of the rocket or ATO unit unpredictable. These difficulties with ammonium nitrate have resulted in the use of double base powders for military rockets and mixtures of special asphalts and ammonium perchlorate for ATO units.

An object of this invention is the preparation of a gas generating composition using ammonium nitrate as the principal gas generating material. Another object is the preparation of a shaped explosive composition (grain) comprising essentially ammonium nitrate, an oxygenated binder and a combustion catalyst, which grain is dimensionally stable and non-fissu-ring in the range between about and 65 F. Still another object is a gas generating composition comprising ammonium nitrate, an oxygenated binder and a combustion catalyst which is suitable for use in rockets and ATO units. Yet another object is the preparation of a composition which is suitable for use as a binder for a mixture or" ammonium nitrate and a combustion catalyst. A further object of this invention is a method for the preparation of said binder. A particular object is a method for the manufacture of a shaped explosive composition which is suitable for use in rockets and ATO units. Still another object is a method of propelling rockets and assisting in the propulsion of aircraft. A further object is a composition suitable for use as a firearm propellant or cannon powder.

FIGURE 1 is a longitudinal view of an ATO unit.

FIGURE 2 is a crossssectional view of said ATO unit taken along the line 2-2.

The above objects and other objects which will become apparent in the course of the detailed description have been achieved as follows. The explosive composition of this invention comprises essentially:

(1) At least about 67 weight percent of ammonium nitrate,

(2) An effective amount of a combustion catalyst, and

(3) Between about 18 and 32 weight percent of a binder,

which binder comprises essentially- (A) Bet-ween about 20 and 50 weight percent of cellulose acetate butyrate polymer which analyzes between about 7 and 55 weight percent of acetic acid and between about 16 and 61 weight percent of butyric acid.

(B) Between about 50 and 80 weight percent of a plasticizer selected from the class consisting of mononitrodiphenyl, dinitrodiphenyl, mixtures of mononitrodiphenyl and dinitrodiphenyl, mixtures of the foregoing with trinitrodiphenyl, mononitrodiphenyl oxide, dinitrodiphenyl oxide, mixtures of mononitrodiphenyl oxide and dinitrodiphenyl oxide and mixtures of the foregoing oxides with trinitrodiphenyl oxide, in which trinitro compound-containing mixtures there is an average of less than about 2.5 nitro groups per molecule and essentially not more than two nitro groups are present on any benzene nucleus.

The term ammonium nitrate as used in this specification and in the claims is int-ended to mean either ordinary commercial grade ammonium nitrate, such as, conventionally grained ammonium nitrate containing a small amount of impurities and which is then generally coated with a small amount of moisture-resisting material such as petrolatum or parafiin, or military grade ammonium nitrate, or a mixture of minor amounts of other inorganic nitrates and ammonium nitrate.

Finely powdered ammonium nitrate contains about 20 volume percent of void space. This void space must be completely filled in order to obtain a shaped explosive grain of the desired physical characteristics. When the combustion catalyst is an organic material, some of the void space is filled by the catalyst. However, when using an inorganic compound as the catalyst, the binder must not only fill the voids of the ammonium nitrate, but also the voids present in the finely powdered inorganic material catalyst. In order to avoid soot formation which leads to a smoky exhaust, it is desirable to have the explosive composition approximately in stoichiometric balance with respect to oxygen content. When using binders that have a high oxygen demand, i.e., are low in bound oxygen content such as hydrocarbons, it has been found that grains approaching the desired characteristics are badly out of oxygen balance. Some lack of oxygen balance due to excess of oxidizable materials is tolerable even though the unbalanced composition has a lower thrust than does the composition that is approximately in oxygen balance. It has been discovered that by the use of oxygenated oxidizable materials as binders for the ammonium nitrate base composition, it is possible to attain the desired physical characteristics and also to attain approximate oxygen balance.

The composition of matter used as the binder in the explosive composition of this invention comprises essentially two components. These components area polymer and a plasticizer. The polymer imparts strength, tear resistance and rigidity to the binder and to the explosive grain. It has been discovered that a particular group in the generic material commonly known as cellulose acetate butyrate possesses the necessary properties for the polymer. The cellulose acetate butyrate used as the polymer in this invention is known as a partially esterified cellulose acetate butyrate and is descr'bed as having an acetic acid content between about 7 and 55 weight percent and a butyric acid content between about 16 and 61 weight percent. The term weight percent acid denotes the amount of acid obtained on saponification of the cellulose acetate butyrate and is expressed as percent of the initial material. A particularly suitable cellulose acetate butyrate is one which analyzes between about 25 and 31 weight percent of acetic acid and between about 31 and 35 weight percent of butyric acid. Commercial grades of cellulose acetate butyrate are described in addition to acid content by its viscosity, when dissolved in acetone. Hereinafter the term viscosity as applied to cellulose acetate denotes the viscosity of an acetone solution containing 20 weight percent of the polymer. The preferred polymer of this invention has a viscosity of between about 10 and 40 centipoises.

Methods of preparing the cellulose acetate butyrate polymer utilized in this invention are given in US. Patents 1,800,860, 2,024,651, 2,030,883, and 2,135,979. Information on cellulose acetate butyrates which are available commercially which are particularly suitable for use in this invention are set out in a brochure of Tennessee- E-astman Corporation, entitled Eastman Cellulose Esters, ninth edition (1950).

A binder having the proper characteristics for use in preparing the shaped explosive composition of this invention contains between about 20 and 50 weight percent of the defined polymer. Preferably, the binder contains between about 27 and 35% of the defined cellulose acetate butyrate.

In order to obtain a plasticizer of the desired thermoplastic characteristics and of the proper solvent action on the polymer, it is necessary to use a compound(s) selected from the class consisting of mononitrodiphenyl, dinitrodiphenyl, mixtures of mononitrodiphenyl and dinitrodiphenyl, mixtures of the foregoing with trinitrodiphenyl, rnononit-rodiphenyl oxide, dinitrodiphenyl oxide, mixtures of mononitrodiphenyl oxide and dinitrodiphenyl oxide and mixtures of the foregoing oxides with trinitr-odiphenyl oxide, in which trinitro compound-containing mixtures there is an average of less than about 2.5 nitro groups per molecule and essentially not more than two nitro groups are present on any benzene nucleus.

It has been found that this plasticizer improves the rigidity and the thermoplastic characteristics of the polymer without seriously affecting the oxygen demand of the binder. The nitrodiphenyls and nitrodiphenyl oxides such as are obtained by the nitration of diphenyl and diphenyl oxide are suitable plasticizers. The trinitrodiphenyls and trinitrodiphenyl oxides are not suitable plasticizers because of low compatibility with the polymer. However, mixtures of nitrodiphenyls and nitrodiphenyl oxides which contain an average of less than about 2.5 nitro groups per molecule are suitable plasticizers for the purposes of this invention. This average nitro content may be attained with mixtures of mononitrodiphenyl (oxide) and trinitrodiphenyl (oxide), dinitrodiphenyl (oxide) and trinitrodiphenyl (oxide), or mixtures of all three nitro compounds. It is preferred to use the dinitrodiphenyls (oxides) and in particular the 2,4-dinitrodiphenyl oxide.

The trinitro derivatives wherein all three nitro groups are present on one benzene nucleus in the molecule are sensitive and tend to explode violently on heating to elevated temperatures. However, very slight amounts of these isomers can be present without rendering the partic. ular nitrodiphenyl (oxide) unusable for the purposes of this invention. Normally the trinitro derivative containing mixtures will contain less than about 1 or 2% of the undesired isomer which contains three nitro groups on a single benzene nucleus.

The binder of this invention contains between about 50 and weight percent of the defined nitro derivatives, and preferably between about 65 and 73 weight percent.

To recapitulate: The composition of matter that is used as a binder for the ammonium nitrate base-explosive of this invention comprises essentially between about 20 and 50 weight percent of a defined cellulose acetate butyrate polymer and between about 50 and 80 weight percent of a defined plasticizer. Preferably, the binder comprises essentially between about 27 and 35 weight percent of polymer and between about 65 and 73 weight percent of plasticizer;

The binder of this invention is preferably prepared by melting the plasticizer to a temperature below about 160 C. and adding to the hot plasticizer the desired amount of polymer. As the plasticizer begins to decompose at temperatures above about 160 C., it is preferred to operate below this temperature and desirably below about 150 C. The mixture is agitated while being maintained at the elevated temperature above the melting point of the modifier, usually between about 100 and 150 C., and the agitation is continued until a substantially homogeneous mixture has been attained. When cooled to ambient temperature the mixture is a very tough, horny material which has thermoplastic properties. The binder is readily converted to a viscous liquid by heating to a temperature above about 100 C.

A mixture of ammonium nitrate and defined binder is quite insensitive and extremely ditficult to ignite at ambient temperatures and pressures. An effective amount of a combustion catalyst must be present in order to obtain a readily ignitable explosive composition with unitorm burning characteristics.

It is well known in this art that the addition of certain types of organic materials will improve the sensitivity of ammonium nitrate. Examples of materials which may be added to improve sensitivity are nitrostarch, nitrocellulose and nitroglycerine. However, only small amounts of these materials can be tolerated without making the composition too sensitive for use as a solid propellant. Materials such as wood flour and sucrose improve the sensitivity of ammonium nitrate-base explosives. Nitrogen containing organic compounds are particularly good for use as combustion catalysts when they do not unduly sensitize the composition. Examples of suitable materials are urea, nitrogu-anidine, 'guanidine nitrate and mononitrate naphthalene. The element sulfur is an effec. tive sensitizer for the ignition of ammonium nitrate. A particularly effective catalyst for the ignition of ammonium nitrate-oxidizoble material mixtures is the element carbon when present in amounts of 6 Weight percent or more. The amount of organic combustion catalyst needed to render the composition readily ignitable and to produce a uniform rate of combustion varies with the particular type of catalyst material added. While at least an efiective amount must be present, it is preferred to keep the amount of organic combustion catalyst at a minimum in order to obtain a grain of proper balance with regard to oxygen stoichiometry and to strength.

The most commonly used method for improving the sensitivity of ammonium nitrate-base explosives is to add a chromiiun containing combustion catalyst. The chromium containing combustion catalysts are ammonium chromate, ammonium polychroniate, alkali-metal chromate, alkali-metal polychromate, chromic oxide, chromic nitrate and copper chromite. The preferred commercial catalyst is ammonium dichromate. The chromium-type catalysts are extremely efiective but have the disadvantage of being expensive and of frequently being virtually unobtainable; furthermore, the chromates are relatively hazardous to handle without the use of special precautions. A more serious disadvantage for military use is the fact that on prolonged storage at somewhat elevated temperatures the chromates tend to react with the oxidizable material in'the composition to give chromate salts which are not as eifective catalytically as ammonium dichromate.

In addition to the above described inorganic chromiumtype catalyst, it has been found that certain organo-cbromium compounds are effective as catalysts. These compounds are disclosed in US. patent application Serial Number 279,968, filed April 1, 1952, now Patent No.

2,997,377, by Wayne A. Proell. These organo-chromiurn compounds are selected from the c'hromate salts of the class consisting of aliphatic polyamines, cycloaliphatic polyamines and alicyclic secondary amines. Examples of these compounds are ethylene diamine chromate, tr'iethy lene tetnamine chromate, hexamethylene diamine chromate, piperidine chromate and dimethyl piperazine chromate.

The arnount of chromium-type catalyst that must be present in the shaped explosive composition of this invention is between about 1 and 8 weight percent and preferably between about 2 and 4 weight percent.

It has been discovered that certain iron compounds are effective catalysts for the combustion of ammonium nitrate and ammonium nitrate-oxidizable material mixtures. These catalysts are the subject matter oi US. patent applications filed by Wayne A. Proell and William G. Stanley, Serial Number 273,564, filed February 26, 1952, now Patent No. 2,987,389, and Serial Number 288,065, filed May 15, 1952, now Patent No. 2,955,033. All of the combustion catalysts disclosed in these applications contain the iron cyanide radical, either ferrocyanide or ferricyanide. In addition to the iron cyanide radical, these catalysts contain a second iron ion which may be either ferric or ferro(us). In addition to the iron-iron cyanide complex, the catalyst may contain alkali metal and/or ammonium ions. It has been found that the generic classes of iron-iron cyanide compounds known as soluble Prussian blues and insoluble Prussian blues are eifective catalysts for the purposes of this invention. It is known that the better soluble Prussian blues contain alkali metal(s) such as potassium and sodium and/or the ammonium radical. Some of the compounds which have been 'found to be effective are: ferro fenrocyanide, ferric ierrocyanide, ferro ferricyanide, ferric ferricyanide, po tassium ferric ferrocyanide, sodium ferric ferrocyanide, ammonium ferric ferrocyanide, potassium soluble Prussian blue, sodium soluble Prussian blue and ammoniumsodium soluble Prussian blue.

These applications show that the so-called insoluble Prussian blues, either the chemical compound ferric ferrocyan-ide, or the commonly known insoluble Prussian blue, are more effective catalysts at high pressure operation than are the soluble Prussian blues. Thus when operating the combustion chamber containing the solid propellant at pressures between about 500 and 2000 p.s.i. a higher burning rate, inches per second, is obtainable when using a given composition containing insoluble Prussian blue as the catalyst than is obtainable when using the same composition using soluble Prussian blue as the catalyst. The insoluble Prussian blue containing explosive compositions are difficult to ignite at atmospheric pressures when the amount of catalyst present is less than about 6 weight percent. However, these compositions ignite readily when an elevated pressure is imposed on the combustion chamber prior to the application of the igniting means.

An application, Serial Number 288.549, filed May 17, 1952, now Patent No. 3,028,273, by Wayne A. Proell, discloses that ammoniated insoluble Prussian blue is an eiiective catalyst for the combustion of ammonium nitnate or ammonium nitrate-oxid-izable material mixtures. The ammoniated insoluble Prussian blue catalyst possesses the ignition characteristics of the soluble Prussian blue catalyst and the burning rate characteristics or the insoluble Prumian blue catalyst. When used in the mixture in amounts of about 2 to 4%, the ammoniated insoluble Prussian blue catalyzed mixture is hard to ignite and does not sustain combustion in an inert atmosphere. The ammoniated insoluble Prussian blue catalyst is produced by exposing an insoluble Prussian blue to the action of ammonia gas. The temperature of the reaction zone containing the insoluble Prussian blue and ammonia gas increases rapidly until a temperature of about 60 C. is reached; as the temperature increases, the rate of increase decreases until at about 60 C. a plateau is reached. A's

measured by temperature increase, the interaction of ammonia and the insoluble Prussian blue is believed to substantially stop when the temperature of the reaction zone I reaches the plateau of about 60 C. The arninoniated insoluble Prussian blue has a strong odor of ammonia after being cooled to room temperature. An 'ammoniated insoluble Prussian blue of catalytic activity about equal to the odorous material which does not possess any appreciable ammonia odor can be obtained by maintaining the odonous material at a temperature of about 70 C. for several hours. The ammoniation of the insoluble Prussian blue does not change the physical appearance of the material and is noticeable principally in that the catalytic activity of the insoluble Prussian blue, particularly at low openatin g pressures, is markedly improved. By this treatment various catalytically active grades of insoluble Prussian blue can be converted to materials having about equal catalytic activity, i.e., the normal variation in catalytic activity of Prussian blue obtained :from different manufacturers can be eliminated by this iammoniation procedure. The term ammoniated insoluble Prussian blue is intended to include both the .ammonia-odorous material and the substantially odor-free material.

An application, Serial Number 287,623, filed May 13, 1952, now Patent No. 3,044,912, by Wayne A. Proell, discloses that alkali metal-iron cyanide and ammoniumiron cyanide :are effective catalysts for sensitizing the ignition and combustion of ammonium nitnate-base explosives. Particularly effective are potassium ferricyanide and ammonium ferricyanide. These iron cyanide catalysts :are not as elfective when used in compositions containing oxygenated oxidizable materials as are the ironiron cyanide complexes, the soluble Prussian blues, the insoluble Prussian blues and the ammoniated insoluble Prussian blues.

When using any one or a mixture of the iron-type catalysts defined above, between about 1 and 8 weight percent of catalyst is present in the explosive composition. Preferably, between about 2 and 4 weight percent of catalyst is used.

The iron-type catalysts are preferred for use in the composition of this invention because of their low cost and ready availability. Further, they do not appreciably increase the heat sensitivity of the ammonium nitrate so that preparation of the shaped grain at temperatures approaching the melting point of ammonium nitrate can be used. Particularly advantageous is the fact that at the normal operating pressures of rocket motors and ATO units the insoluble Prussian blues and ammoniated insoluble Prussian blues give higher burning rates than do the other iron-type catalysts and the chromium-type catalysts. Still another advantage for the iron-type catalyst is the fact that the residue of the iron-type catalyst is a fine black powder producing substantially no smokiness in .the combustion gases Whereas the combustion residue of the chromium-type catalyst is a light, fiuify material which imparts smokiness to the combustion gases.

It is to be understood that the catalyst consumes some of the excess oxygen produced in the decomposition of the ammonium nitrate and this consumption of oxygen must be considered in determining the total oxygen demand of the explosive composition.

The explosive composition of this invention comprises essentially ammonium nitrate, a binder and a combustion catalyst. The ammonium nitrate is present in an amount of at least about 67 weight percent. An elfective amount of combustion catalyst must be present and when using either a chromium-type catalyst or an iron-type catalyst, between about 1 and 8 weight percent, preferably about 2 .to 4 weight percent of catalyst is present in the explosive composition. The amount of binder present in the explosive composition is between about 18 and 32 weight percent; preferably, the amount of binder should be adjusted to give a composition that is approximately stoichiometrically balanced with respect to oxygen.

The explosive composition of this invention can be prepared by several methods. A preferred procedure is set out below. The binder is heated until it has become a viscous fluid at a temperature on the order of to 120 C. The desired amount of ammonium nitrate in a finely powdered condition is admixed with the desired amount of catalyst also in a finely powdered condition; this mixing is preferably carried out at ambient temperatures and precautions are taken to prevent the ammonium nitrate reaching a temperature approaching the melting point due to the friction of the mixing. The ammonium nitrate-catalyst mixture is slowly added to the fluid binder while agitating the materials. The temperature of the mixture is preferably maintained below about 120 C. in order to minimize the possibilities of heat ignition of the composition. The materials are mixed until a smooth homogeneous paste has been obtained. This paste may be permitted to cool to ambient temperatures and then broken up into irregular pieces for uses such as blasting powder. Preferably, the pasty mass is formed into shapes of a configuration suitable for solid propellant purposes. The configurations are commonly called grains. The forming may be by introducing the pasty mass into suitable molds, either manually or mechanically by means of an injection molding technique, or preferably by extrusion. The composition of this invention flows fairly readily at temperatures above about C. and becomes dimensionally stable at temperatures below about 90 C. This characteristic simplifies the problem of forming the grain because it eliminates the necessity for accelerated cooling of .the mold or the extruded grain. The composition of the explosive of this invention has a very large advantage over conventional solid propellant compositions. It has been found that defective grains can be reworked alone or in admixture with fresh materials to produce satisfactory grains. The reworkability of re jects results in a substantial saving over conventional grains where rejects cannot be reworked.

It has been discovered that the extrudibility of the composition of this invention can be greatly improved without adversely affecting the characteristics of the shaped composition. It has been found that the pressure needed to extrude a tubular grain having an outside diameter of 0.25 inch could be decreased from about 2000 p.s.i. to about 1000 p.s.i. by adding between about 0.2 to 1 weight percent of an alcohol, such as, heptanol, octanol, nonanol, steryl, etc.

The drawings show a particular application of this invention to an assisted take-off unit. The ATO unit illustrated is designed to be hung under the wing of an aircraft; normally at least two units, i.e., one under each wing, will be used. In FIGURE 1, the body of the unit is made up of a tubular member 11 which is closed at one end and which is provided with threads at the open end. Member 11 is provided with .two loops, 12 and 13. These loops are used to hang the unit from a carrier, not shown, which is attached to the Wing of the aircraft. This carrier makes it possible to jettison the unit after take-oif. A somewhat funnel shaped member 14 is attached to member 11 by engagement of the threads at the large open end of member 14 with the threads on member 11. Member 1-4 is provided with a nozzle 16 through which the decomposition products pass, The size or nozzle 16 determines in part the pressure maintained inside of the chamber formed by members 12 and 14.

The solid propellant fills the cylindrical portion of member 11. The solid propellant in this illustration consists of seven tubular grains, 17, l8, 19, 2t 21, 22, and 23; each having an OD. of about 3 inches and having a centrally located cylindrical opening 1 inch in diameter, the full length of the grain; the grains are approximately 30 inches long. The grains used herein consist essentially of 2% of ammoniated insoluble Prussian blue catalyst, 24% of binder and 74% of ammonium nitrate.

1 1 the hexagonal grain is dependent on the gas evolution rate desired.

The following information is presented to illustrate the preparation of the binder composition and the shaped explosive composition; the elfectiveness of the explosive composition for gas generation; and the resistance of the shaped explosive composition to fissuring when subjected to temperatures over the range of about 100 F. to +17 P.

All of the test grains presented herein were made with a binder that had been prepared as follows:

The polymer was a commercial cellulose acetate butyrate purchased from Tennessee Eastman and listed as CAB 272-20. This cellulose acetate =butyrate analyzed between 28 and 29 weight percent of acetic acid and between about 32 and 34 weight percent of butyric acid; the viscosity of the standard acetone solution was between -15 and 35 centipoises at 25 C.

The ammonium nitrate used was technical grade and had a particle size-Rotap analysis-as follows:

The catalyst was finely pulverized in a Mikro pulverizer in order to permit more uniform distribution with the ammonium nitrate.

The explosive composition was prepared in the following sequence of steps. The desired amount of plasticizer Was placed in an agitated vessel and heated to 150 C. The desired amount of polymer was added to the vessel and the materials maintained at a temperature of about 150 C. until a homogeneous viscous mass had been obtained. (The binder as prepared above was quite thermoplastic in nature and could be cooled to room temperature to give a very tough, horny solid and then reconverted into a viscous fluid by heating to an elevated temperat-ure.)

The molten binder was cooled to 120 C. and then a mixture of the required amounts of ammonium nitrate and catalyst were added to the container. The contents of the vessel were stirred While the temperature was maintained between about 110 and 120 C. until a uniformly mixed pasty mass had been obtained. This pasty mass was cooled to about 100 C. and then shaped into the desired configurations by various methods. The composition described above was somewhat plastic at a temperature of about 100 C. (212 F.) but quickly became rigid at temperatures below 90 C. and was quite dimensionally stable at 85 C. (183 F.)

Cylinders of about 1 inch diameter and about 1.5 inches long were prepared by the use of a hand press. The required amount of material to make a dense cylinder was placed into a steel mold having a 1 inch inside diameter and about 50 pounds of pressure was applied on the material through a close-fitting plunger. The cylinder-s prepared in this way were dense and had a smooth, hard surface. These cylinders were quite strong and could stand a considerable amount of rough treatment.

Large size hollow grains were prepared for use in a miniature rocket motor. These grains as molded were 8 inches long, 2.75 inches in diameter and had a 1 inch longitudinal coaxial perforation. The 2.75 inch grain was machined to an outside diameter of 25 inches for use in the rocket motor. The composition machined readily on an ordinary lathe to give a grain of a smooth, very hard surface. This large perforated grain was prepared by the use of a steel mold which was provided with a 1 inch 'steelcore. The desired amount of explosive composition was prepared and maintained at a temperature of 120 C.; by the use of an oven the mold was heated to 120 C. The required amount of material was manually placed into the mold and tamped into place with a wooden paddle. A steel disc 2.75 inches in diameter was then inserted into the open end of the mold and by means of a hydraulic piston, about lbs. pressure was applied to the disc in order to compact the explosive material. The mold and contents were allowed to cool gradually to about 50 C.; at this temperature the grain was removed from the mold. The grain was permitted to cool gradually to room temperature and was then machined to a 2.5 inch outside diameter and was sawed into 4 inch lengths.

For burning rate tests, grains about 6 inches long and 0.25 inch in diameter were prepared by extrusion. A laboratory extrusion device was prepared. A chamber for the explosive was adapted to be maintained at a temperature of betweeen 100 and C. A hydraulic piston was used to force the explosive from the chamber through an 0.25 inch die to give a grain of the proper diameter. The hydraulic piston could develop pressures up to 2000 p.s.i. it was found that by using this device strands 30 inches long could be prepared readily.

The burning rates in inches per second of various compositions were determined at elevated pressures by the use of a Crawford bomb. This device permits measuring the burning rate of a strand of material at a constant pressure in the combustion chamber. The bomb is brought to the desired pressure by the use of cylinder nitrogen and the strand is ignited by means of a hot wire. Duplicate runs were made in order to determine the reproducibility of the burning rates. Although there is an appreciable change in burning rate with increase in pressure, normally burning rates are reported at 1000 p.s.i. operating pressure since this is about the operating pressure of ATO units.

The resistance of the particular composition to fissuring on temperature change was determined by a laboratory method. No standard test has as yet been established by testing laboratories and the test described below was developed by the applicants and is believed to. be a good indication of the cycling resistance of the particular composition.

Apreliminary screening procedure utilized solid cylinders of 1 inch diameter and 1.5 inch length. The grain was placed in a glass bottle containing a small amount of drying agent; the drying agent prevents the condensation of water on the grain at Dry Ice temperatures. This bottle was enclosed in a second glass bottle. The glass-enclosed grain was placed in a thermostatically controlled oven and maintained for 4 hours at an oven temperature of 70 C. (+158 R). The glass-enclosed grain was then removed and held at ambient temperature about 25 C. for 1 hour. Then the glass-enclosed grain was buried in Dry Ice for 3 hours; Dry Ice temperature is -80 C. (-112 F). The glass-enclosed grain was removed and allowed to come to an ambient temperature; the grain was removed from the glass bottles and inspected for cracks on the surface. (It has been noted that grains fail by cracks that appear on the surface of the grain. Failure by internal fissuring has not occurred in the grain configurations tested herein.)

The grain was cycled to failure or for eight cycles, whichever occurred first. Cycling tests indicate that passage of eight cycles is a sufiicient indication that the grain will cycle indefinitely.

The large perforated grains were tested as described above except that instead of using glass bottles the grain was protected by the use of a polyethylene bag. For the large grain it appears that successful completion of eight cycles indicates satisfactory resistance to fissuring upon temperature change. it has been found that grains which successfully pass eight cycles on the 1 inch cylin- Each grain has the annular area at each end inhibited against burning by a coating of asphalt in order that the burning may take place on the cylindrical surfaces only. For some uses it is desirable to have a grain which burns cigarette fashion in which case the outer surface and one end of the grain will be inhibited to prevent combustion. The inhibiting means may be asphalt, cellulose acetate, ethyl cellulose, acrylic plastics, etc.

Although a tubular grain is illustrated herein, the invention is not limited to such a grain. Any particular shape may be utilized. Examples of other shapes are cylinder, cruciform, trifor-m, hexaform, octaform and slab. In the case of perforated grains, the perforation may be circular or star-shaped with various numbers of points in the star. Furthermore, a single cylindrical grain having one or more longitudinal perforations may be utilized in some cases, instead of the multigrain unit shown.

The grains are held in longitudinal position and prevented f-rom sliding back and forth in the combustion chamber by means of a wire grid 26. Wire grid 26 consists of a ring cut to fit the threads of member 14 and provided with a grid work of metal wires that will resist the high temperature existing in the combustion chamber.

On one side of the conical portion of member 14 the-re is provided for the combustion chamber a safety venting means 28. Venting means 28 comprises a tubular member fastened to member 14, which tubular member has full access to the combustion chamber and is provided with a rupture disc, not shown. The rupture disc is of such construction that excess pressure in the combustion chamber will blow out the disc, whereby the pressure in the combustion chamber will be held below the point of serious damage to the unit.

An igniter means is positioned within member 14 so as to close off the nozzle 16. The igniter means consists of a container 31 filled with black powder, or some other easily ignited material, which can produce a large volume of gases at elevated pressure. A squib 32 for igniting the powder, is attached to the container 31 in communication with the powder contained therein. Electrical wires 33 connect a wire in the squib to the electrical system of the aircraft and a switch therein (the connections to the aircraft are not shown).

The ATO unit is assembled as follows: The grains are inserted into member 11. Venting means 23 are fastened to member 14. Igniter 31 is inserted through the large open end and fitted so as to close the nozzle, the wires 33 having first been passed through the nozzle 16. The wire grid 26 is screwed into the large open end of member 14. The assembled nozzle portion is then securely screwed onto member 11.

The assembled unit is then attached to the wing of the aircraft by loops 1?. and 13; wires 33 are connected to the electrical operating assembly in the aircraft. When the pilot desires to obtain the assisted takeoff, he throws the switch which causes the current to pass through wire 33 and \to heat up the firing wire in squib 32, which in turn ignites the powder in the container 31.

The container is of sufficient strength to withstand the initial pressure generated by the gases from the powder. The hot gases raise the pressure in the combustion chamber high enough to permit the grain to ignite. The combustion of the grain causes the pressure in the chamber to rise to a point which cannot be resisted by container 31. The container disintegrates and the pieces are discharged through nozzle 16. The total time from throwing the switch to full operation of the unit is on the order of less than 0.5 second.

As the gases pass out of the nozzle the reaction acts on the aircraft and adds its thrust to assist the aincnafts propeller; a marked increase in forward speed results and permits the aircraft to take off in a shorter space of time or it permits lifting a load heavier than could be airborne by the use of the propellers alone.

When using about 4% or more of the iron-type catalyst, the grain will ignite at relatively low pressures and no special precautions are necessary to maintain elevated pressure in the chamber until ignition occurs, as shown above. In this case, the igniter may be introduced into the combustion chamber by a means attached to the conical portion of member 14 (this procedure is conventionally used and is illustrated in US. 2,479,828) and no closure is placed on nozzle 16.

The conventional placement of the igniter may be used with the lower catalyst content grains. However, it is necessary to use a much heavier powder charge in the igniter or, preferably, the nozzle is provided with a rupture disc, which is set to blow out at about 500 p.s.i. Other methods of igniting the grain can be readily devised.

While the invention has been illustrated by means of an assisted take-off operation, it must be understood that the solid propellant of this invention can also be used for other purposes. Some of these are airto-air missiles, ground-to-ground missiles, blasting powder, etc. An important use of the invention lies in the production of gases at elevated pressures in a stationary or a portable system; discontinuous operation is readily obtained when using about 2% of catalyst as the composition can be extinguished readily by merely depressuring the combustion chamber.

An extremely important use of the composition of this invention lies in the field of propellent powder for small arms ammunition, artillery powder and so-called cannon powder. Small Mme ammunition is intended to include cartridges for pistols and rifles and shells for shotguns. Artillery ammunition is intended to include the selfcontained ammunition, i.e., a shell wherein the projectile and the casing that contains the powder and igni-ter are in one piece, much like -a rifle cartridge. For very large caliber, artillery pieces such as mm. antiaircraft guns mid 6 inch or greater naval guns, separate ammunition is used, i.e., the projectile and the propelling powder are separate. Usually the powder is contained in special cloth bags.

One of the most serious problems in artillery weapons is the extremely rapid wear of the bore, particularly when firing is at a continuous high rate. The commonly used propellant, baillis-tite, has a flame temperature of about 5900 R, which temperature softens the barrel and permits the projectile to erode away the rifiing quite rapidly. Not only is the composition of this invention much cheaper than ballistite, but also the much lower flame temperature of about 3900 F. permits a much longer useful life of the rifled barrel. Another desirable feature of this composition is that it is virtually smokeless.

The propellant for use in small arms is normally used in the form of short, solid thread-like filaments varying in length from about 0.1 to 0.2 inch. These grains are obtained by forcing the pasty powder through a multiple hole die to form filaments which may be between about 1 and 5 mm. in diameter. These filaments are chilled by an air blast in order to form long threads of propellant. The threads are broken into short lengths and graded by a screening operation. The diameter of the filament-like grain controls the gas evolution rate when the grain is ignited. For some uses instead of a solid thread or spaghetti grain, a perforated macaroni-type grain may be used. This macaroni-type grain is particularly useful for smaller caliber artillery ammunition. For separate ammunition use it has been found that grains which are hexagonal in external outline and are provided with longitudinal perforations are preferable because the hexagonal shape permits the formation of large multiple grains of a honeycomb structure, which grains can be readily fitted into a cloth bag of the required diameter for the particular gun. These perforated hexagonal grains may be as much as l or 2 inches in diameter and several inches long. The number of perforations in der test also pass eigh-t cycles on the large perforated grain test.

v The grains that passed the cycling test were tested for internal fissuring by burning at ambient temperature and pressure. Smooth burning indicates no fissures are present as fissures cause uneven burning, i.e., sudden increases in gas evolution result when a fissure is reached.

In order to simulate larger scale operation, a miniature rocket motor was constructed. This motor consisted essentially of a cylinder closed at one end and threaded at the open end. The straight side of the cylinder was about 8 inches long and the cylinder had an internal diameter of about 3 inches. A funnel-shaped portion pro vided with an opening for the attachment of a nozzle and provided with threads at the larger end was threaded onto the cylindrical casing to complete the combustion chamber of the motor. Various sized orifices were provided in order to permit the operation of the motor at different combustion chamber pressures. These orifices varied from 0.17 to 0.24 inch in diameter. By varying the orifice size, the combustion chamber pressure could be varied from about 700 to about 2000 p.s.i.

In order to reduce the amount of explosive material needed per motor test, a perforated cylindrical aluminum slug, 2.5 inches in diameter and 4 inches long, was used to take up about half the longitudinal volume of the motor. Thus in operation the motor contained the slug and a 2.5 inch diameter perforated grain 4 inches long.

The grain was ignited by a black powder mixture, which mixture was in turn ignited by means of an electrical squib. It was found that satisfactory ignition could be obtained by using 25 g. of the following mixture: sparkler powder, 10 g.; FFG gun powder, 7.5 g.; and FFFG powder, 7.5 g. This mixture was placed at the nozzle end of the funnel-shaped member and was held in place by means of a paper disc pressed firmly against the sloping sides of the member. The motor was assembled by inserting into the casing first the aluminum slug and then the grain to be tested. The test grain was inhibited on both annular ends by coating the annular area with asphalt. This inhibiting of the an-nuli results in the burning of the cylindrical surfaces only. The nozzle end complete with powder igniter was then screwed to the casing. The electrical squib was then inserted through the nozzle opening until it contacted the powder mixture. This method of arming the motor is particularly desirable because the motor is essentially inert until a few seconds before the test run is fired. It has been found that this method of ignition gives ignition delays between about 10 and 500 milliseconds.

Grain l The binder for this grain consisted of 31% of CAB 272-20 and 69% of 2,4-dinitrodiphenyl oxide. The grain consisted of binder, 21.5%; ammonium nitrate, 74.5%; magnesium "nitrate, 2%; and ammonium dichromate catalyst, 2%. This grain successfully passed 8 cycles in the 1 inch cylinder test and the cycled grain burned smoothly showing that no internal fissuring had occurred.

Grain II This grain was identical with Grain I except that the catalyst was a soluble Prussian blue. This grain successfully passed 8 cycles in both the 1 inch cylinder test and the large grain test. The burning rate in the Crawford Bomb at 1000 p.s.i. was 0.115 inch per second. The calabout 20 and 50 weight percent of cellulose acetate butyrate polymer which analyzes between about 7 and weight percent of acetic acid and between about lo and 61 weight percent of butyric acid and (B) between about 50 and 80 weight percent of a plasticizer selected from the class consisting of mononitrodiphenyl, dinitrodiphenyl, mixtures of mononitrodiphenyl and dinitrodiphenyl, mixtures of the foregoing with trinitrodiphenyl, mononitrodiphenyl oxide, dinitrodiphenyl oxide, mixtures of mononitrodiphenyl oxide and dinitrodiphenyl oxide and mixtures of the foregoing oxides with trinitrodiphenyl oxide, in which trinitro compound-containing mixtures there is an average of less than about 2.5 nitro groups per molecule and essentially not more than two nitro groups are present on any benzene nucleus.

2. The composition of claim 1 wherein said combustion catalyst is ammonium dichromate.

3. The composition of claim 1 wherein said combustion catalyst is present in an amount between about 1 and 8 weight percent and said catalyst consists of at least one member of the class consisting of iron-iron cyanide complexes, soluble Prussian blue, insoluble Prussian blue, ammoniated insoluble Prussian blue, ammonium iron cyanide and alkali-metal iron cyanide and mixtures thereof.

4. The composition of claim 1 wherein said catalyst is ammoniated insoluble Prussian blue and wherein said catalyst is present in an amount between about 1 and 8 weight percent.

5. The composition of claim 1 wherein said polymer analyzes between about 25 and 31 weight percent of acetic acid and between about 31 and 35 weight percent of butyric acid.

6. The composition of claim 1 wherein said plasticizer is dinitrodiphenyl.

7. An explosive composition which comprises essentially (l) at least about 67 weight percent of ammonium nitrate, (2) between about 2 and 4 weight percent of a combustion catalyst selected from the class consisting of iron-iron cyanide complexes, soluble Prussian blue, insoluble Prusian blue, ammoniated insoluble Prussian blue, ammonium iron cyanide, alkali-metal iron cyanide and mixtures thereof, and (3) between about 18 and 32 weight percent of a binder, which binder comprises essentially (A) between about 27 and 35 weight percent of cellulose acetate butyrate polymer which analyzes between about 25 and 31 weight percent of acetic acid and between about 31 and 35 weight percent of butyric acid and (B) between about 65 and 73 weight percent of a plasticizer selected from the class consisting of mononitrodiphenyl, dinitrodiphenyl, mixtures of mononitrodiphenyl and dinitrodiphenyl, mixtures of the foregoing with trinitnodiphenyl, mononitrodiphenyl oxide, dinitrodiphenyl oxide, mixtures of mononitrodiphenyl oxide and dinitrodiphenyl oxide and mixtures of the foregoing oxides with trinitrodiphenyl oxide, in which trinitro compound-containing mixtures there is an average of less than about 2.5 nitro groups per molecule and essentially not more than two nitro groups are present on any benzene nucleus.

8. The composition of claim 7 wherein said plasticizer is dinitrodiphenyl oxide.

9. The composition of claim 7 wherein said catalyst is ammoniated insoluble Prussian blue.

10. The composition of claim 7 wherein said composition is about stoichiometrically balanced with respect to oxygen.

11. The composition of claim 7 wherein said plasticizer is 2,4-dinitrodiphenyl oxide.

References Cited in the file of this patent UNITED STATES PATENTS 2,434,872 Taylor Jan. 20', 1948 2,568,080 McGahey Sept. 18, 1951 2,592,623 Turnbull Apr. 15, 1952 

1. AN EXPLOSIVE COMPOSITION WHICH COMPRISES ESSENTIALLY (1) AT LEAST ABOUT 67 WEIGHT PERCENT OF AMMONIUM NITRATE, (2) AN EFFECTIVE AMOUNT OF A COMBUSTION CATALYST, AND (3) BETWEEN ABOUT 18 AND 32 WEIGHT PERCENT OF A BINDER, WHICH BINDER COMPRISES ESSENTIALLY (A) BETWEEN ABOUT 20 AND 50 WEIGHT PERCENT OF CELLULOSE ACETATE BUTYRATE POLYMER WHICH ANALYZES BETWEEN ABOUT 7 AND 55 WEIGHT PERCENT OF ACETIC ACID AND BETWEEN ABOUT 16 AND 61 WEIGHT PERCENT OF BUTYRIC ACID AND (B) BETWEEN ABOUT 50 AND 80 WEIGHT PERCENT OF A PLASTICIZER SELECT FROM THE CLASS CONSISTING OF MONONITRODIPHENYL, DINITRODIPHENYL, MIXTURES OF MONONITRODIPHENYL AND DINITRODIPHENYL, MIXTURES OF THE FOREGOING WITH TRINITRODIPHENYL, MONONITRODIPHENYL OXIDE, DINITRODIPHENYL OXIDE, MIXTURES OF MONONITRODIPHENYL OXIDE AND DINITRODIPHENYL OXIDE AND MIXTURES OF THE FOREGOING OXIDES WITH TRINITRODIPHENYL OXIDE, IN WHICH TRINITRO COMPOUND-CONTAINING MIXTURES THERE IS AN AVERAGE OF LESS THAN ABOUT 2.5 NITRO GROUPS PER MOLECULE AND ESSENTIALLY NOT MORE THAN TWO NITRO GROUPS ARE PRESENT ON ANY BENZENE NUCLEUS. 